The present invention generally relates to the field of electrically controllable impedance and/or reactance devices. More particularly, the present invention relates to the field of tunable radio receivers in which radio tuning is at least partially accomplished by the use of an electrically tunable inductor.
Generally, electrically tunable inductors include a control winding wound on a magnetically saturable core with a signal winding also wound on the same core. A control current is passed through the control winding in order to control the degree of magnetic saturation of the core. The inductance of the signal winding is inversely related to the degree of magnetic saturation of the core, which in turn controls the small-signal permeability of the core. Thus by altering the magnitude of the control currents passing through the control winding, the magnitude of the inductance of the signal winding is varied.
In general, radio receivers using such a controllable inductor for tuning purposes have been disclosed (see U.S. Pat. No. 2,941,173). However, C.C.I.s have not been used for tuning radio receivers which are tunable only in discrete frequency increments or require a consistent tuner reset capability. This is because C.C.I.s have a substantial hysteresis effect. In other words, for any one control current, at least two different values of inductance are possible because of the residual magnetism effect of the magnetic core of the C.C.I. Thus electronically tuning the RF stage of a radio through the use of a C.C.I. has not been previously possible in pushbutton radios or any other radio in which the tuning is accomplished in predetermined discrete steps. For these types of radios either mechanically controllable inductive tuning systems comprising coils and mechanically slidable cores or electrically controllable capacitive tuning systems using varactor diodes have been used. The mechanically tunable inductive systems are complicated, expensive and subject to mechanical wear which will eventually effect their accuracy. The capacitive varactor tuning systems are particularly inappropriate for the tuning of the antenna input stage of an automotive AM receiver (550 kHz through 1600 kHz). This is because automotive AM broadcast band antennas are generally much shorter than a quarter wavelength and therefore act as a capacitive reactance. When a subsequent tuned RF stage is tuned by a varactor over the broadcast band, it can be shown that the gain of the tunable RF stage will vary greatly as it is tuned over the band. Inductively tuning the RF stage will eliminate this undesirable gain variation. However, as previously mentioned because of the hysteresis characteristics of C.C.I.s this type of electronic tuning was not possible when the tuning was to take place in discrete frequency increments or in a controlled repeatable manner.